Radio bomb release system with warning period



May 18, 1948. D. BLITZ RADIO BOMB RELEASE SYSTEM WITH WARNING PERIOD Filed April 25, 1945 2 Sheets-Sheet 1 TIME/ NVENTOR.

nrmswn May 18, 1948. D. BLITZ RADIO BOMB RELEASE SYSTEM WITH WARNING PERIOD Filed April 25, 1945 2 Sheets-Sheet 2 Illll ll EN TOR. partial fi y egg A Patented May 18, 1948 RADIO BOMB RELEASE SYSTEM WITH WARNING PERIOD K Daniel Blitz, Princeton, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application April 25, 1945, Serial No. 590,184

6 Claims. (01. 343-7) This invention relates to radio bomb release systems, and more particularly to improvements in systems of the type described in copending U. S. application Serial No. 524,794, filed March 2, 1944, by Royden C. Sanders, Jr., and William R. Mercer, and entitled Radio bomb release system, and which has matured into Patent No. 2,412,632, dated December 17, 1946.

Systems of this type operate by measuring the range and speed of a bombing craft with respect to a preselected target, and efi'ecting bomb release upon the attainment of the relationship of range and speed which will result in a hit. This relationship depends upon the altitude of the bombing craft, as described more fully hereinafter.

The system may be adjusted automatically in accordance with the altitude in order to permit the pilot to fly at any altitude within certain limits, rather than at apredetermined fixed altitude. Nevertheless it is necessary that the bomber craft be maintained in level flight at a constant altitude for a short period, of the order of one second, before the release occurs.

It has been found in practice that the pilot tends to anticipate the operation of the automatic release, releasing the bomb manually (and prematurely) under the impression that the automatic equipment has failed to function. Also, since the pilot can only guess at the time remaining until release, he is forced to cease evasive maneuvers and fly level for a period much longer than the one second required for accurate operation.

Accordingly, it is the principal object of the present invention to provide a system of the described type including means for automatically warning the pilot that a bomb is about to be released, and indicating that the equipment is operating.

The invention will be described with reference to the accompanying drawings wherein:

Figure 1 is a diagram showing the geometry of a'bomb release problem,

Figure 2 is a graph showing the relationship of slant range to slant speed for bomb release at a particular altitude, and the linear approximation to said relationship,

' Fi ure 3. is a schematic circuit diagram of a radio bomb release system embodying the present invention,

The geometry of bomb release Refer to Figure 1. It is assumed that an aircraft at the point P is flying horizontally at a velocity G toward a point M, directly over a target at the point Q, at an altitude H. A bomb released at the altitude H without any vertical velocity will require a time T1 to fall to the level of the target.

where g equals the acceleration caused by gravity, 32.2 feet/sec, approximately.

' In order for the bomb to strike at the point Q, it must be released T; seconds before the craft reaches the point M. The time T which must elapse before the craft reaches the point M is where D equals the distance from P to M.,

If the horizontal speed G and the horizontal component D of the target range were known, the calculation of T would be a simple matter. The condition for release is:

T=Tf, or substituting (l) and (2),

' D 2H (3) Ft T Radio reflection equipment does not measure the horizontal distance D, but the true or slant distance R. Similarly, the horizontal speed G is not measured directly, but its slant component S is measured. Accordingly, it is necessary to determine the time T in terms of these quantities. It is apparent from Figure 1 that at great distances or low altitudes from the target, the target depression angle a. will be small, and the slant speed and slant range will be nearly the same as the horizontal speed and horizontal range. As the distance is decreased or the altitude increased, the differences between the .slant and horizontal speeds and distances will increase.

l3, I5, l6 and I1.

circuit 21.

Referring to 'Figure 2, the curvel shows the relation between slant speed 8 and slant range R which corresponds to one particular value of T1. which in turn is the time of fall T from an altitude H1. Thus if a bomb is released from the altitude H1, when R and S are of such values as to satisfy the relationship represented by the curve I, the bomb will strike the target.

For every different altitude, there is a dlflerent relationship between R and S which must be satisfied for proper release. .The curve I is thus but one of a family of similar curves. In the present system a linear approximation is used, rather than the actual R-S relationship. This approximation need be accurate only within the range S to S" of slant speeds which will occur in the practical operation of the system. The dash line 3 in Figure 2 is the linear approximation to the curve I which produces the smallest maximum error throughout the range of slant speeds from S" to S.

The equation of theline 3 is (4) R=mS+R where m is the slope of the line:

and R is the range intercept at zero speed, as indicated in Figure 2. As stated above, the relations shown in Figure 2 correspond to one specified altitude H1. For any other altitude, both m and R0 will have correspondingly different values. By setting in the values of m and R0 corresponding to the particular altitude H at which a craft is flying, a substantially correct release may be obtained by satisfying the relationship of Equation 4.

Radio system and computer circuits Refer to Figure 3. A radio transmitter 5 is provided with an antenna 1, and is connected to a frequency modulator 9, which may be of the type described in copending U. S. patent application Serial Number 471,003, filed January 1, 1943, by S. V. Perry, and entitled Capacity modulator unit, or any other known device for varying the frequency of operation of the transmitter 5 in accordance with the voltage applied to it. In the present illustration, the modulator 9 is energized in such manner as to provide triangular wave frequency modulation of the transmitter I.

A D.-C. source II is connected across resistors An adjustable resistor 25 is connected across the resistor I5. A switch I9 is connected across the resistor I1, and is ar ranged to be cyclically opened and closed by a cam 2 I continuously'driven by a motor 23. This periodically changes the voltage at the junction point Id of resistors I3 and I5 between two values which differ by an amount depending upon the setting of the resistor 25. A doublethrow contact assembly MI is connected to shortcircuit the resistor I6 when operated to its right hand position, and to shunt a resistor I8 across the resistor I5 when operated to its left-hand position. When the resistor I6 is shorted, the amplitude of variation of the voltage at the point I4 is that which would be required for normal operation of the system without warning. When the resistors I 6 and I8 are included in the circuit, the amplitude is decreased.

The point I4 is connected to a wave shaping integrating circuit, or may be of the type de- The circuit 21 may be merely anscribed in copending U. S. patent application Serial No. 546,537, filed July 25, 1944, by Royden C. Sanders, Jr., and entitled Wave shaping circuits and which has matured into Patent No. '2,403,616, dated July 9, l946. The circuit 21 converts the square wave voltage input from the point I4 to a substantially triangular wave voltage, which is applied to the modulator 9. A resistor 29 is included in the input circuit 'of the wave shaping circuit 21. The band width through which the frequency of the transmitter 5 is swept is determined by the adjustment of the resistor 25, and by the setting of the contact assembly I.

A receiver 3| is provided with an antenna 39, and is coupled to the transmitter 5 through a transmission line 35. is connected to the output of the receiver II. The output circuit of the limiter 31 is connected to a pair of frequency responsive circuits comprising averaging cycle counters 39 and H respectively. The counter 39 includes a capacitor 43, connected from the limiter 31 to the anode of a triode 45 and to the cathode of a diode 41. The cathode of the triode 45 is connected to the control grid of a cathode follower amplifier tube 49. The entire load resistance 5I associated with the tube 49 is connected in its cathode circuit so that it acts as a so-called ,cathode follower. The anode of the diode 41 is connected to a tap 53 on the resistor 5I.

The counter 4I includes a capacitor 55 connected from the limiter 31 to the anode of a triode 51 and to the cathode of a diode 59. The anode of the diode 59 is connected to the control grid of the cathode follower tube 49. The cathode of the triode 51 is connected to the upper end of the load resistor 5I, at the cathode of the tube 49. The counters 39 and H are provided with a common storage capacitor 6|, connected between the control grid of the.tube 49 and ground. They also have a common load resistor 63, connected from the grid of the tube 49 to a point 55 on a voltage divider chain described hereinafter.

The control grid of the triode 45 of the counter 39 is effectively coupled to the ungrounded end of the resistor I1, so that a square wave voltage is applied thereto in synchronism with the frequency modulation of the transmitter 5. The control grid of the triode 51 is coupled to the same point through a phase invertor 91, so that the square wave voltage applied thereto is out of phase with that at the grid of the triode 45. The connections are such that the triode 51 is cut off and the triode 45 is conductive while the frequency of the transmitter 5 is increasing, and the triode 45 is cut ofl and the triode 51 is conductive while the transmitted frequency is decreasing. A

The cathode of the cathode follower tube 49 is connected to the cathode of a relay amplifier tube 69. The anode circuit of the tube 69 includes the actuating coil of a relay 1|. The contacts of the relay 1| are connected to the warning system described hereinafter. The control grid of the relay tube 69 is bypassed to the cathode through a capacitor 13, and is connected through a resistor 15 to the adjustable contact of a variable voltage divider 11.

The voltage divider 11 is shunted across a portion of a voltage divider chain including fixed resistors 19, 8|, 82, 83, 84 and 85, and variable resistors 81 and 89, serially connected across the D.-C. source II. A switch I43 is connected across An amplitude limiter 31 the resistor 84. A double-throw switch I45, ganged with the switch I43, connects the upper end of the voltage divider TI to the adjustable contact of the resistor 81 when the switch I43 is 'open, and to the lower end of the resistor 82 when the switch I 43 is closed. The voltages at the terminals of the voltage divider 11 are controlled by the variable resistors 81 and 89, and by the positions of switches I43 and I45. The potential at the control grid of the relay tube 69 is variable between these two voltages by adjustment of the voltage divider TI. The return point 65 of the counter load resistor 63 is the Junction between the fixed resistors 83 and 85.

Operation of radio system and computer Assume that switches I iI, I43 and I45 are all in their right-hand positions, removing resistors I 6 and I8 from the sweep circuit and includin resistors 8d and 81 in the relay bias circuit. Owing to the cyclical operation of the switch I9 by the motor 23, the frequency of the-output of the transmitter varies cyclically as shown by the solid line II of Figure 4. Some of this output is transferred directly to the receiver 3i through the line 35. The greater portion is radiated by the antenna I. Some of the radiated energy strikes the selected target (not shown), and is reflected to the antenna 33. The time required for the transmitted energy to travel to the target and back to the antenna 33 is roportional to the slant range R from the aircraft to the target. The variations in frequency of the received energy are accordingly delayed with respect to those of the transmitted energy. The variation of frequency of the received signal as a function of time, assuming no relative motion between the aircraft and the target, is shown by the dot line I03. The delay Tr is proportional to the range R.

Now if the range is decreasing at a rate S, the frequency of the received signal will be increased, owing to to Doppler effect. Thus the frequency of the received signal will vary with time as shown by the dash line I05. The increase in frequency Fa is directly proportional to the speed S.

The direct and reflected signals from the transmitter 5 are mixed and detected in the receiver 8i. The output of the receiver comprises a beat frequency signal, which has a frequency equal to the difi'erence in the frequencies of the two signals applied to the receiver. The frequency of this beat signal varies with time as shown by the solid line graph I06 of Figure 5. The mean value of the beat frequency, indicated by the dash line I01 in Figure 5, is directly proportional to the range R, and is equal to:

fmftR 246 where fin is the modulating frequency in cycles per second, fa is the sweep width, or range of where in is the mean transmitted frequency (see Figure 4) in cycles per second, S is the slant speed in feet per second, and c is the velocity of wave propagation (the velocity of light) in feet per second. During decrease of transmitter frequency the beat frequency is fllfl 2&3

As mentioned above, the trlcde 51 of the counter I is cut off during increase of transmitted frequency, and the triode of the counter 39 is conductive. During'this period, the counter 38 operates to provide an average current is through the load resistor-88 in the direction of the solid arrow. This current is:

where in is a constant directly proportional to the capacitance of the capacitor 43. During this period the counter 4| does not operate.

During decrease of transmitted frequency, the

trlcde 45 is cut 011 and the counter 39 does not The triode 51 is conductive, and the operate. counter a l provides an average current is through the load resistor 63 in the direction of the'dash arrow. This current is:

where so is the potential at the point 65 and R is the resistance of the load resistor 63.

The cathode of the tube 49 is maintained at substantially the same potential as the control grid as long as the current through the resistor 5| is only the anode current of the tube 69. Thus the potential at the cathode of the relay tube 69 is the same as that at the grid of the tube 49. The potential at the control grid of the relay tube (referred to ground) is the voltage e1 at the tap of the voltage divider 17. Thus the voltage es between the cathode and the control grid of the relay tube is:

Rearranging the terms,

Rf.(k1 -k) 1%) -s k, k2)

The quantities k1, k2, R1., in and in are all constants, determined in accordance with design considerations. Therefore Equation 12 can be written as:

( 3= 0' i+ lf| K2S where el-a) and As long as ea is suflicie ntly large to bias the relay tube 69 to cutoif, no current flows through the relay 1| and it remains open. However, when e: becomes equal to e'a, the voltage at which the tube 53 starts to conduct, the relay II is closed. At this time oi+KIf.R The slant range is therefore:

.K= rrw (14) K1f. Tm. This may be expressed as Equation 4 above,

where and I (16) Ro= f e 8 Thus by setting the values of K e e '-e' In. mid Tu? in accordance with the altitude H,'the relay can be made to operate at the proper release time, within the limits of the linear approximation of the corresponding R-S curve.

It is apparent from Equation that m is inversely proportional to the bandwidth is. Accordingly, the proper value of m for any particular altitude H may be obtained by adjustment of the sweep width control 25. R0, as shown by Equation 16, is also inversely proportional to the bandwidth, and is directly proportional to the voltage which must be present across the counter load resistor 63 to cause the relay tube 69 to conduct. This voltage is the difierence between the total bias from cathode to grid of the relay tube 69, (eo-e1) and the cutoff bias 6'3, and is a function of the settings of the variable resistors 89, 81 and II, which control the bias e1 at the grid of the relay tube 69. The variable resistor 89 is employed to control R0 in accordance with the altitude.

The variable resistor 11 is provided to allow variation of the bias e1 on the relay tube for the purpose of obtaining an adjustable range lead, 50 that release may be made to occur a certain distance in advance of the target. Since the voltage required for a, given range lead will vary with altitude, the variable resistor 81- is included to set the voltage across the resistor 11 as a function of altitude. Thus a given setting of the resistor 11 will provide a given range lead, regardless of the altitude.

The controls 81, 89 and 25 are ganged on a shaft III! in order that the modulation bandwidth and the bias voltages may be adjusted simultaneously to correspond with the altitude. None of these quantitiesare linear functions of the altitude. The resistors 81 and 89 are designed with resistance-rotation characteristics comprising two linear portions of -diiiferent slopes to provide approximately the required variations of voltage with rotation of the shaft III). Practically, the errors introduced by this arrangement are negligible.

The modulation bandwidth must be held within about one percent of the correct value. While this could be achieved with a tapered variable resistor, there would be difficulty in constructing for a counter circuit I25.

such a device, and it would necessarily be large in order to obtain the required accuracy. On

the other hand, it is relatively easy to make a a variable resistor of reasonable size having an accurately linear resistance-rotation characterlstic. By proper proportioning of the resistors I3, I5, I1 and 23, the variable resistor 25 may be made linear and yet provide the correct characteristic of bandwidth vs. shaft position.

Automatic altitude compensation The shaft IIII may be set manually to a posirelay II3 to a power source H5. The actuating coil of the relay H3 is connected in the anode circuit of a relay amplifier tube I I1, like the relay tube 83. The control grid of the tube II! is coupled to a bias source comprising an adjustable voltage divider IIB connected in series with a resistor I2I across the source II. This bias is applied to the tube III through a resistor I23.

The resistor I23 comprises the load resistor The counter I25 is connected to the output circuit of an amplitude limiter I21, which in turn is connected to the output circuit of a receiver I29. A transmitter I3I, including means for cyclically varying the transmitted frequency, is coupled to the receiver I29 and is provided with an antenna I33. A similar antenna I35 is connected to the receiver I29.

The counter I25, limiter I21, receiver I29,

and transmitter I3I comprise a radio reflection altimeter, and cooperate in known manner to provide a voltage across the load resistor I23 bearing a predetermined relationship to the alt tude H, and in opposing polarity to the bias voltage provided by the voltage divider I I9. When the algebraic sum of this voltage and the voltage at the adjustable tap of the voltage divider H9 is negative with respect to the cutoif voltage of the relay tube III, the relay I I3 is deenergized. This connects the motor to the source I I5 in such polarity as to rotate the shaft III! in the direction corresponding to increase of altitude. The tap of the voltage divider is rotated to produce more positive bias, until the voltage across the counter load resistor is neutralized. The relay tube III starts to conduct, energizing the relay H3 and disconnecting the motor III from its source H5.

The relay H3 is provided with a small dead space so that a slight increase of energization is required to close the upper contacts. Thus if the voltage across the counter load resistor I23 decreases, the relay will operate to its upper position, energizing the motor I I I to rotate the shaft III! in the direction corresponding to decrease of altitude. Thus the shaft H0 is maintained substantially continuously at a position corresponding to the altitude.

, Warning system It has been assumed throughout the above description that the switches MI, I43 and I45 have all remained in their right-hand positions so that the sweep width and the relay bias were ad- Initially, however, the switches I4I, I43 and I45 are In their left-hand positions, as shown in the drawing. The sweep width is is narrower than that indicated by Equation 14 and the relay tube bias e1 is lower. This causes the system tooperate the relay H at a time from target T+Tw, where Tw is the warning time. The length of the period Tw depends upon the values of resistors I6, I8, 8! and 84. It is preferred at present to make Tw of the order of three or four seconds.

The contacts 01 the relay 1I, instead of being connected directly to the bomb release mechanism, as in the above-mentioned systems, are

connected to a system of sequentially operating relays I41, I49 and HI. The relay I41 includes the contacts I4I connected to the sweep circuit, already described, the contacts I43 and I45 connected to the bias circuits, and two further sets of contacts I53 and I 55. The contacts I53 and I55 are open when the relay I 41 is deenergized, the contacts I4I, I43 and I45 are in their lefthand positions.

The relay I 49 includes contact sets I51 and I59.. These are in their left-hand positions until the rela I49 is energized. The relay I5I includes contact sets I IiI and I63, both of which are normally open, and are closed upon energization of the relay I5I.

An alarm device I65, which may be a lamp, a buzzer, or other indicator is connected inparallel with the actuating magnet of the relay I41. Energy for operating the alarm I65 and the relays I41, I49 and I5l is supplied by a source I61,

through a manually operable switch I69.

In the operation of the system, the movable contact of the relay H is initially in its lower position and the relays I41, I49 and I5I are not energized. As a target is approached, the radio and computer system operates as described above, energizing the relay 1I, Tw seconds before the predicted release time.- The relay I41 and the alarm I65 are energized through the switch I69, the contact I51 of the relay I49, and the upper contact of the relay 1|.

The alarm I65 starts and continues to operate throughout the bomb release period. The contacts I53 close, setting up a holding circuit for the relay I41. The contacts I4! operate to exclude resistors I6 and I8 from the sweep circuit, increasing the sweep width and contacts I43 and I45 operate to reset the bias on the relay tube 69. The contact I55 is closed.

The increase in sweep width .and the bias change recalibrate the system to operate'the relay H at a time from target, T. The relay 1| is then deenergizd as a result of the recalibration of I the system, completing a circuit through the winding of the relay I49 and contacts I55. The contacts I59 close, setting up a holding circuit for the relay I49. The contacts I 51 operate to transfer the connection of the upper contact of the relay H to the winding of the relay I5l.

Nothing further happens until the bomber craft reaches the point at which release should occur. At this time, the relay H is energized again, closing the circuit through the coil of the relay I5I and the contacts I51. Contacts I63 close, setting up a holding circuit for the relay I 5I. Contacts [6| close, actuating the bomb release mechanism. The function of the relay I5I is merely to latch the circuit so that any momentary chattering of the relay H at the time of release will not cause erratic operation of the release mechanism. It is evident that the release mechanism could be connected directly to efiect bomb release.

The invention has been described as an improved bomb release system of the radio reflection type wherein signals radiated from an aircraft to a selected target are picked up after .refiection thereby, and utilized tofirst provide a warning that bomb release is imminent, and to release a bomb shortly thereafter. This is accomplished by initially setting the calibration of the equipment so as to operate before release should occur. Instead of releasing the bomb, this operation activates a warning signal and simultaneously changes the calibration to the correct one for release. The equipment then operates a second time to efiect the release.

I claim as my invention:

l. A radio bomb release system including a relay, radio speed and distance measuring means responsive substantially continuously to the distance and speed of a bombing craft with respect to a' selected target to effect initial operation of said relay upon the attainment of a predetermined relationship between said distance and speed corresponding to a time from target T-I-Tw, wherein T. is the time from target at which a bomb should be dropped from said craft to strike said target, and Tw is a warning period; means responsive to said initial operation of said relay to effect a warning indication, a second relay responsive to said operation of said first relay to recalibrate said radio speed and distance. measuring means to operate said first relay a second time upon the attainment of a second predetermined relationship between said distance and speed corresponding to the time from target T, and a third relay responsive to said second operation of said first relay to efi'ect bomb release.

2. A radio bomb release system including a relay, radio speed and distance measuring means responsive substantially continuously to the dis tance and speed of a bombing craft with respect to a selected target to effect operation of said relay upon the attainment of a predetermined relationship between said distance and speed corresponding to a time from target Tel-T10, wherein T is the time from target at which a bomb should be dropped from said craft to strike said target, and Tw is a warning period; means responsive to said operation of said relay to effect a warning indication, and further relay means responsive to said operation of said first relay to recalibrate said radio speed and distance measuring means to operate said first relay a second time upon the attainment of a second predetermined relationship between said distance and speed corresponding to the time from target T, and. means responsive to said second operation of said relay to 3. A radio bomb release system including a relay, radio speed and distance measuring means responsive substantially continuously to the distance and speed of a bombing craft with respect to a selected target to effect operation of said relay upon the attainment of a predetermined relationship between said distance and speed corresponding to a time from target T+Tw, wherein T is the time from target at which a bomb should be dropped from said craft to strike said target, and Tw is a warning period; means responsive to said operation of said relay to effect a warning indication, and means responsive to said operation of said relay to recalibrate said radio speed and distance measuring means to operate said relay a second time upon the attainment of a secnd predetermined relationship between said dis-v tance and speed corresponding to the time from target '1", and means responsive to said second operation of said relay to effect bomb release.

4. A radio bomb release system including, on a bombing craft, transmitter means for radiating a signal to a selected target, means for cyclically varying the frequency of operation of said transmitter means, means for receiving said signal after reflection by said target, frequency responsive means differentially responsive to the output of said receiver means during increase and decrease respectively of the frequency of said transmitter, and relay means responsive to the output of said frequency responsive means to operate upon the attainment of a predetermined magnitude of the output from said frequency responsive means, corresponding to a predetermined relationship between the distance and speed of said craft with respect to said target; means normally maintaining the range of frequency variation of said transmitter at such value as to cause initial operation of said relay a period Tw seconds before the time from target T at which a bomb should be released from said craft to strike said target, further relay means responsive to said initial operation of said first-mentioned relay to alter said range of frequency variation and to bias said first relay means so as to cause a second operation of said first relay at the time from target T, an alarm device connected to said further relay means, and third relay means responsive .to said second operation of said first relay to release a bomb.

5. A radio bomb release system including, on a bombing craft, transmitter means for radiating a signal to a selected target, means for cyclically varying the frequency of operation of said transmitter means, means for receiving said signal after reflection by said target, frequency responsive means differentially responsive to the output of said receiver means during increase and decrease respectively of the frequency of said trans- 1 mitter, a relay responsive to the output of said frequency responsive means to operate upon the attainment of a predetermined magnitude of the output from said frequency responsive means, corresponding to a predetermined relationship between the distanceand speed of said craft with respect to said target; means normally maintaining 12 the range of frequency variation of said transmitter at such value as to cause initial operation of said relay a period Tw seconds before the time from ,target T at which a bomb should be released from said craft to strike said target, means responsive to said initial operation of said relay to eflect an alarm indication, a second relay responsive to said initial operation of said first relay to alter said range of frequency variation to a value such as to cause a second operation of said first relay at the time from target T, and means responsive to said second operation of said first relay to release a bomb.

6. A radio bomb release system including, on a bombing craft, transmitter means for radiating a signal to a selected target, means for cyclically varying the frequency of operation of said transmitter means, means for receiving said signal after reflection by said target, frequency responsive means differentially responsive to the output of said receiver means during increase and decrease respectively of the frequency of said transmitter, and relay means responsive to the output of said frequency responsive means to operate upon the attainment of a predetermined magnitude of the output from said frequency responsive means, corresponding to a predetermined relationship between the distance and speed of said craft with respect to said target; means normally maintaining the range of frequency variation of said transmitter at such value as to cause initial operation of said relay a period Tw seconds before the time from target I at which a bomb should be released from said craft to strike said target, means responsive to said initial operation of said relay to effect an alarm indication and to alter said range of frequency variation to a value such as to cause a second operation of said relay at the time from target T, and means responsive to said second operation of said relay to release a bomb.

- DANIEL BLITZ.

REFERENCES CITED UNITED STATES PATENTS Name Date Bradley Apr. 30, 1946 Number 

